The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of acquiring imaging data at more than one energy range using a multi-energy imaging source modulated at multiple-view intervals.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
A CT imaging system may include an energy discriminating (ED), multi energy (ME), and/or dual energy (DE) CT imaging system that may be referred to as an EDCT, MECT, and/or DE-CT imaging system. Such systems may use a scintillator or a direct conversion detector material in lieu of the scintillator. The EDCT, MECT, and/or DE-CT imaging system in an example is configured to be responsive to different x-ray spectra. For example, a conventional third generation CT system may acquire projections sequentially at different peak kilovoltage (kVp) levels, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. Energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy.
Techniques to obtain the measurements comprise: (1) scan with two distinctive energy spectra, and (2) detect photon energy according to energy deposition in the detector. EDCT/MECT/DE-CT provides energy discrimination and material characterization. For example, in the absence of object scatter, the system derives the behavior at a different energy based on the signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
A principle objective of dual energy scanning is to obtain diagnostic CT images that enhance contrast separation within the image by utilizing two scans at different chromatic energy states. A number of techniques have been proposed to achieve dual energy scanning including acquiring two scans either (1) back-to-back sequentially in time where the scans require two rotations around the subject, or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials. High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two dual energy images may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as required with previous CT technology. Additionally, taking separate scans with several seconds apart from one another results in mis-registration between datasets caused by patient motion (both external patient motion and internal organ motion) and different cone angles. And, in general, a conventional two-pass dual kVp technique cannot be applied reliably where small details need to be resolved for body features that are in motion.
Therefore, it would be desirable to design an apparatus and method for acquiring imaging data at more than one energy state during a single scan having minimum patient motion and minimized cone angle effects.